Process for the manufacture of paraformaldehyde



Aug. 12, 1969 R. SCHUMACHER PROCESS FOR THE MANUFACTURE OF PARAFORMALDEHYDE Filed May 19, 1967 CONDENSER I0 I BUBBLE CAP RECTIFICATION 2 N3 MR SE NB A M H C ,PLATES 22 COLUMN 22 BUBBLE CAP '3 PLATES 2m AQUEOUS J FORMALDEHYDE SOLUTION 2 GATES 6 INVENTOR Roif Schumacher ATTORNEYS 'U.S. Cl. 260-6155 United States Patent M 3,461,171 PROCESS FOR THE MANUFACTURE OF PARAFORMALDEHYDE Rolf Schumacher, Berlin, Germany, assignor to Karl Fischer, Apparate-u. Rohrleitungsbau, Berlin-Borsingwalde, Germany Filed May 19, 1967, Ser. No. 639,671 Int. Cl. C07c 47/04 1 Claim ABSTRACT OF THE DISCLOSURE The continuous rectification of aqueous formaldehyde solutions under reduced pressure, to produce paraformaldehyde therefrom, is carried out in two stages wherein the formaldehyde solution is fed to a first rectifying column operated at a pressure of 100-150 mm. Hg or higher, and the resulting vapor mixture is fed to a second column operated at considerably lower pressure of about 20-30 mm. Hg to yield a strongly concentrated paraformaldehyde product.

Background of the invention The present invention concerns a process for the manufacture of paraformaldehyde by the continuous rectification of aqueous formaldehyde solutions under reduced pressure.

It is known in the prior art to perform the rectification of aqueous formaldehyde solution under reduced pressure in a column, into which the formaldehyde solution is fed in its middle portion, and in which the drying of the concentrate takes place in a drying apparatus located in the lower end of the column which serves as a sump.

The apparatus and method of the prior art are unsatisfactory in most respects. This is principally attributable to the fact that the nature and behavior of the system formaldehyde-water has not been adequately investigated, so that suitable process steps have not been developed.

Under normal pressure, the system formaldehyde-water has an azeotropic point. Under reduced pressure this point disappears, and it is therefore possible to obtain a correspondingly formaldehyde-poor condensate at the top of the column, the lower the absolute pressure is at that point. Pressure and temperature stand in parallel relationship. Since the pressure in the sump of the column is only slightly higher than that in the top or head of the column, the temperature in the sump is also only slightly higher.

When an attempt is made to obtain a condensate as poor as possible in formaldehyde by maintaining a low working pressure at the head of the column, it follows that in the sump of the column the pressure and the temperature likewise are relatively low.

Under these conditions, however, there is a danger that insufiiciently concentrated product collecting in the sump will polymerize at comparatively low temperatures, thus thickening, and blocking the apparatus, especially the heating arrangement, such as the vaporization tubes. It was therefore diflicult or practically impossible, prior to the present invention, to achieve an optimum, since when the column is operated throughout at higher temperature into the sump, there is obtained a more strongly concentrated, and warmer, sump product, with less tendency toward premature polymerization, but at the same time, at the warmer head of the column there is obtained a condensate which is too rich in formaldehyde and hence uneconomical.

3,461,171 Patented Aug. 12, 1969 Summary of the invention In accordance with the present invention, the manufacture of paraformaldehyde by the continuous rectification of aqueous formaldehyde solutions is performed in such manner that only very small and unavoidable losses of formaldehyde take place in the removal of the distillation water vapor, while at the same time a strongly concentrated paraformaldehyde is obtained, which is not prematurely polymerized and which does not contain substantial amounts of water, and which requires only a relatively minor after-drying.

Thus, the present invention concerns a process for the manufacture of paraformaldehyde by continuous rectification of aqueous formaldehyde solutions under reduced pressure, whereby the rectification is performed in two successive connected stages. In the first stage, aqueous formaldehyde solution is fed to a rectifying column operated at a higher pressure, such as 100-150 mm. Hg, and the vapor mixture collected at the top or head of the first column is introduced into the lower end of a second column which is operated at a considerably lower pressure of about 20-30 mm. Hg. The foregoing pressure ranges are not to be considered as limiting, however, for in accordance with another aspect of the present invention, it has been found that by increasing the pressure employed in the first stage of the process to more than 150 mm. Hg, there is an advantage in efiiciency in that the resulting higher operating temperature results in a more strongly concentrated final product, thereby substantially decreasing the load on the after-drying apparatus, such as, for example, a two-roll vacuum evaporator.

Brief description of the drawing The practice of the process of the present invention will be better understood by reference to the accompanying drawing showing a schematic arrangement of a twocolumn rectification system, together with its auxiliaries.

Description of the preferred embodiments In the drawing, a first rectification column 21 having bubble cap plates 21a is fed at about its mid-point through inlet tube 2 with a 30-40% by weight aqueous formaldehyde solution. At the lower end of column 21, the socalled sump 23, there is connected a conventional tu-be vaporizer 4, which is operated as a heat source in such manner that an operating pressure of about -150 mm. Hg is maintained in the sump 23. There is also connected to sump 23, via conduit 23, a two-roll vacuum evaporator apparatus 5, in which a concentrate containing 55%85% formaldehyde, fed to the evaporator from the lower end of the first column through conduit 23', is dried to a scaly product (paraformaldehyde) containing 82%98% formaldehyde by weight, which is removed through reversible vacuum gates 6-. Drum or roller vacuum apparatus, such as that illustrated as element 5 in the drawing, is standard equipment known in the art, as described, e.g. by E. L. Holland-Merten, Handbuch der Vakuumtechnik (Handbook of Vacuum Engineeing), Chapters VIII and XII to XV, published by Wilhelm Knapp, Halle (Saale), D.D.R., 1950.

The vapors formed in the two-roll vacuum evaporator 5 are returned in sump 23 in the lower part of the first column via pipe 7, thus supplementing tube vaporizer 4 as an additional source of heat.

The formaldehyde-water vapor mixture formed at the upper end of first column 21 is led through conduit 21' to the lower end of a second column 22 where bubble cap plates are shown at 22a. The latter is operated at a considerably lower pressure, which is so regulated that at the head 28 of the second column it amounts to about 20-30 mm. Hg. The pressure drop in passing from first column 21 to second column 22 can be produced by any suitable valve arrangement of a type commonly employed in connection with apparatus of this type. Solely for purpose of illustration, there is shown in the drawing at the lower end of the second column 22 an expansion chamber 32.

A portion of the liquid-vapor mixture contained in second column 22 is withdrawn from the lower end of the column and recirculated to the upper end of the first column 21 via conduit 32 by means of pump 40, which is capable of overcoming the selected pressure drop.

The vapors collecting at the head 28 of column 22 are led through pipe 9 into condenser 10. A definite portion of the condensate thus obtained, which contains only about 0.1% to 0.5% by weight of formaldehyde, may be vented through pipe 11, while another portion can be fed back into the head 28 of the column 22 via pipe 12.

As mentioned previously, the first column can be operated at a pressure higher than 150 mm. Hg, and while there is no particular upper limit of this pressure, experience has shown that a practical working upper limit of pressure is of the order of about 500 mm. Hg.

The operating temperatures in the respective columns are about 55-98 C. in the first column, and about 22- 35 C. in the second column, depending upon the pressure.

The concentration of the aqueous formaldehyde feed is about 3'0%40% formaldehyde by weight, but may be varied within wide limits.

I claim:

1. A process for the manufacture of scaly paraformaldehyde by the continuous rectification of aqueous formaldehyde solutions under reduced pressure comprising the steps of distilling aqueous formaldehyde solution in a first rectification column at a pressure of from mm. to about mm. mercury and a temperature of from about 55 to 98 C., feeding formaldehyde-water vapor mixture from said first column to a second rectification column at its lower portion and maintaining in said sec ond column a pressure of about 20 mm. to 30 mm. mercury and a temperature of from about 22 to 35 C., wherein aqueous formaldehyde solution is supplied to said first column at about its middle portion, formaldehydewater vapor mixture is withdrawn from the head of the first column, a portion of the liquid-vapor mixture is withdrawn from the lower end of said second column and recirculated to the head of said first column, concentrate from the lower end of the first column is fed into a vacuum drum evaporator, vapors of which are recirculated to the lower part of said first column to supply additional heat thereto, and wherein scaly paraformaldehyde is recovered from the vacuum evaporator.

References Cited UNITED STATES PATENTS 2,527,654 10/1950 Pyle et al. 2,527,655 10/1950 Pyle et al. 2,529,622 11/1950 Michael 260--615.5 XR 2,568,017 9/1951 MacLean et al. 260-615.5 XR 2,578,338 12/1951 Craven 260615.5 XR 2,581,881 1/1952 Pyle et al. 3,113,972 12/1963 Kodama et al. 260-6155 XR LEON ZITNER, Primary Examiner HOWARD T. MARS, Assistant Examiner US. Cl. X.R. 20375, 77 

